Generally, an illumination of an interior space such as an office and so on uses a plurality of fluorescent lights installed on a ceiling. The fluorescent light as a light source is comparatively cheap, but a life time thereof is comparatively short, and the longer a period of use is, the lower a brightness thereof is.
In addition, a radiation angle, which is a radiated angle of a light, of the fluorescent light is equal to or larger than 120 degrees, and the fluorescent light illuminates the whole of the interior space in a regular illuminance.
In order to solve problems of the fluorescent light as the light source having the above-mentioned problems such as the comparatively short life time and the decrease of the brightness according to the use thereof, technologies for using an LED, of which a life time is very long and a power consumption is low, as an interior illuminating device have been developing.
For example, in Registered Patent No. 10-1052457, a lamp using the LED as the light source illuminates the interior space by widening the radiation angle of a light radiated from the LED to an angle equal to or larger than 120 degrees by using a diffusion plate or a lens.
However, the above-mentioned conventional method for illuminating a space cannot obtain an effect of decreasing energy largely, although the conventional method for illuminating a space uses the LED of which efficiency is high. Thus, an LED plane light-emitting device for replacing the conventional fluorescent light is suggested, in which the LED plane light-emitting device illuminates a corresponding interior space entirely, but a relationship between a horizontal illuminance and a vertical illuminance is not considered in the LED plane light-emitting device.
Hereinafter, the conventional method for illuminating a space is described in more detail with reference to accompanying drawing.
FIG. 1 is an illustration for describing the conventional method for illuminating a space.
Referring to FIG. 1, the conventional method for illuminating a space sets the vertical illuminance and the horizontal illuminance similarly by using a plurality of adjacent illuminations L1 and L2 of which radiation angles are equal to or larger than 100 degrees.
That is, the whole of the interior space is illuminated by setting an illuminance of a horizontal surface of an object and an illuminance of a vertical surface of the object to be almost equal in a specific position.
In FIG. 1, for convenience of description, light progression paths A1 to A4 and B1 to B4 of each of the illuminations L1 and L2 within the radiation angle are shown. All of the illuminations L1 and L2 are equally positioned at a height of h from a bottom surface.
At this time, in illuminance measuring points P1 and P4 in a vertical direction from each of the illuminations L1 and L2, since the light progression paths A1 and B1 are shortest, an illuminance is the highest when a single illumination is installed, but on the contrary, the nearer to an edge of the radiation angle, the lower an illuminance of the bottom surface is.
This is because the illuminance is inversely proportional to a square of a distance from the light source.
At this time, in an illuminance measuring point P1 in the vertical direction, the light progression path A1 of the illumination L1 and the light progression path B4 of another illumination L2 overlap, and thus the illuminance of the illuminance measuring point P1 of a corresponding position is determined by an amount of the light emitted from two illuminations L1 and L2.
In addition, the illumination measuring point P2 is a position where the light progression path A2 of the illumination L1 and the light progression path B3 of the illumination L2 overlap, and the illuminance is determined according to the distances from the two illuminations L1 and L2.
The progression paths affecting the two illuminance measuring points P1 and P2 are A1 and B4, and A2 and B3, respectively, and at this time the light amount has A1>A2>B3>B4 relation, thus the illuminances of two illuminance measuring points P1 and P2 are substantially equal.
This may be equally applied to all illuminance measuring points P1, P2, P3 and P4.
As described above, the whole space illuminated by the two adjacent illuminations L1 and L2 has a regular illuminance due to the effect of the two adjacent illuminations L1 and L2.
In addition, in FIG. 1, when a random illuminance measuring point P between the two illuminations L1 and L2 is considered, the radiation angles of the lights radiated from each of the illuminations L1 and L2 is wide, and thus the vertical illuminance which is the illuminance of the vertical surface of the random illuminance measuring point P and the horizontal illuminance which is the illuminance of the horizontal surface of the random illuminance measuring point P are almost equal.
Thus, a whole of an upper side and a lower side of the space is seen brightly. At this time, when the upper side of the space should be dark and a work space (i.e. the lower side of the space) should be bright, in order to achieve this, an additional auxiliary illumination should be furnished.